The Senate was expected to vote as soon as this week on physics professor Ernest Moniz's nomination to lead the Energy Department. The Senate Energy Committee endorsed Moniz in a 21-1 vote last week.

South Carolina Republican Sen. Tim Scott cast the only dissenting vote. Like Graham, Scott opposes Obama's plan to cut about $200 million from a project to turn weapons-grade plutonium into nuclear reactor fuel. The plant is being built at the Savannah River nuclear site.

A spokesman confirmed that Graham has placed a "hold" on the nominee and plans to meet with Moniz.